Epoxy compositions and their curing techniques are well known, and the patents issued on curable epoxy compositions number in the hundreds. It will be appreciated that each and every one of the known epoxy-curing systems exhibits advantages over other systems, and, as importantlyj, disadvantages over the same system. There is, of course, a continuing need to develop better epoxy compositions.
There is, in particular, an increasing need in the automotive sector for high performance thermosetting compositions of matrices for fiber reinforced composites. Fiber reinforced composites are very desirable in automotive applications because they can offer a combination of good stiffness, strength and are light weight. Increasingly, the automotive manufacturers have demanded higher performance from the thermoset resins used in fiber reinforced composites. These higher performance thermoset resins are expected to possess these following characteristics:
good mechanical properties at temperatures up to about 90.degree. C. PA0 good thermal oxidative stability PA0 good toughness properties, including good impact resistance PA0 good fatigue properties PA0 good chemical and solvent resistance PA0 good fire resistance PA0 high resistance to humidity, e.g., the "hot-wet" properties of the composite must remain high. PA0 (b) "Y" represents a flexible segment comprising stiff units (SU') and optional flexible units (FU') which stiff units and flexible units are interconnected; PA0 (c) said stiff units, SU and SU', are independently selected from the group consisting of substituted and non-substituted aromatic rings, cycloaliphatic rings and heterocyclic rings; PA0 (d) said flexible units, FU and FU', are independently selected from the group consisting of ##STR2## (e) the number of stiff segments in said molecules is "a", the number of flexible segments in said molecules is "b", and the ratio of ##EQU1## is between zero and one; (f) the number of stiff units and flexible units are selected such that the average number of total stiff units ##EQU2## divided by the average number of total flexible units ##EQU3## is less than or equal to four and; PA0 (a) a first component selected from the group consisting of phenol compounds of the formula HO--X--OH or HO--Y-OH, where X and Y represents the stiff segment and flexible segment respectively, defined above; with PA0 (b) a second component, said second component being a diepoxide selected from the group consisting of ##STR3## and PA0 Glass transition temperature, Tg=121.degree. C. (DSC) PA0 Fracture toughness, K.sub.q =2.4 KSI.sqroot.in (Compact Tension) PA0 Flex modulus, E=370 KSI (Dry @ R.T.) PA0 Impact Strength=4.8 ft.lb/in PA0 Water gain, .alpha.W/W.sub.o =2.5% (saturation)
A further, and very important property of such systems is that the composite must have acceptable processing characteristics. For example, the current technique for manufacturing certain automotive components typically involves the use of liquid processes such as resin transfer molding (RTM). Therefore, the most desirable thermosetting resin compositions should be processable on the standard equipment currently utilized in the automotive industry.
A broad spectrum of thermosetting epoxy resin systems is currently being used by the automotive industry, primarily as composite matrices and adhesives. As a class, epoxy resin systems are very versatile materials offering, as mentioned above, chemical resistance, high adhesive strength, good electric properties and are easy to use or process into composites. However, to improve their high temperature properties, such current epoxy resin systems must be highly crosslinked. This crosslinking, however, results in generally lower toughness.
One good resin system possessing improved impact is based on Shell Chemical Company EPONOL.RTM. Resins. These thermoplastic polyethers are disclosed and claimed in U.S. Pat. No. 3,637,590, while the process for preparing such polymers is claimed in U.S. Pat. No. 3,306,872. Even though such polymers, which are based on the reaction product of certain diepoxides with certain bisphenols, have improved impact strength, such polymers lack adequate high temperature properties and solvent resistance for high performance applications. Similar compositions having improved impact strength are also disclosed in Fed. Rep. of Germany OLS No. 2,142,579 where certain diepoxides are reacted with certain diphenols (e.g., 2,2-bis(4-hydroxynaphth-1-yl)propane) to produce polymers for eyeglasses.